Thread: define operator<< for std::pair<string, int>

The following code seems working for the built in type, but not for the pair. Can anybody help me out? Thanks.

Code:

#include <iostream>
#include <iterator>
#include <map>
using namespace std;
template<class T>
void print2darray(const int m, const int n, const T& a) {
	for (int i = 0; i < m; i++) {
		for (int j = 0; j < n; j++) {
			cout << a[i][j] << " ";
		}
		cout << endl;
	}
}

typedef pair<string, int> mypair;

ostream& operator<<(ostream& s, const mypair& p) const{
	return s << "(" << p.first << ", " << p.second;
}

int main() {
	int a[2][2] = { { 1, 2, }, { 3, 4 } };
	double b[2][2] = { { 1.0, 2.0 }, { 3.0, 4.0 } };
	string c[2][2] = { { "as", "it" }, { "mine", "your" } };
	pair<string, int> d[2][2];
	for (int i = 0; i < 2; i++) {
		for (int j = 0; j < 2; j++) {
			d[i][j] = make_pair(c[i][j], a[i][j]);
		}
	}
cout << "array a[2][2]:" << endl;
print2darray(2,2,a);

cout << "array  b[2][2]:" << endl;
print2darray(2,2,b);

cout << "array c[2][2]:" << endl;
print2darray(2,2,c);

cout << "array d[2][2]:" << endl;
print2darray(2,2,d);

}

Your problem is the definition of operator<<() for your mypair type. It cannot be a const-qualified function, as it is not a member of a class.

It is generally a good idea not to rely on "using namespace std;" for resolving types of arguments in function definitions (potential for ambiguity if other namespaces and using directives are in play). Declare ostream as std::ostream explicitly.

You probably also need to add a closing ")" to the end of the output.